once you get in the house your golden just open the map and look at the guards patrols and sneak in between them you may need to fibrewire some of them then kill the mark grab his car keys and steal his car for the exit

It took me ages to beat this mission, but when I finally did I got Silent Assassin on pro . I Fiberwired the guard on his loo break, went into the basement entrance and up the stairs in the kitchen to the top floot. Go out onto the balcony here and you can walk around the roof to the marks office and fiberiwre him from behind, before leaving the way you came.